The invention relates to a method of depositing doped polycrystalline silicon, hereinafter referred to as polysilicon, at low temperature.
Thin doped polysilicon films are typically deposited at temperatures between 540 and 625 degrees C. by low pressure chemical vapor deposition (LPCVD.) Alternatively, an amorphous silicon film can be deposited at lower temperatures with dopants, then annealed after deposition at temperatures of 550 degrees C. or greater to crystallize the silicon and activate the dopants. In general, as deposition temperature drops, the deposition rate and quality of doped polysilicon films decreases.
The temperatures required to create doped polysilicon films using conventional methods are incompatible with other processes and materials that may be desirable. For example, aluminum metallization withstands a maximum practical fabrication temperature of only 475 degrees C. for semiconductor processing.
Ishihara, U.S. Pat. No. 5,956,602, xe2x80x9cDeposition of Polycrystal Si Film,xe2x80x9d discloses a method to deposit a doped polysilicon film at temperatures below 500 degrees C. This method introduces a source gas like SiH4 and a dopant gas such as BCl3, PH3, or Al(CH3)3. The two gases are flowed at different times, without overlapping. Hydrogen plasma is then used to anneal the film. The process is repeated until a film of the desired thickness is produced.
While this method claims to produce doped polysilicon at temperatures below 500 degrees C., it has disadvantages. Repeatedly introducing the source and dopant gases at different times and annealing with hydrogen plasma involves significant process complexity and requires specialized equipment. Further, alternately introducing the silicon and dopant source gases may result in dopant nonuniformities throughout the film.
There is a need, therefore, to create high-quality doped polysilicon films at temperatures less than those used in conventional processes and that do not require a subsequent high temperature anneal. It would be preferable if complex deposition cycles and specialized equipment are not required.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. In general, the invention is directed to a method to deposit low-resistivity doped polysilicon at low temperatures.
According to one aspect of the invention, a method for depositing a doped polysilicon film comprises providing a surface and substantially simultaneously flowing SiH4 and BCl3 over the surface at a temperature less than or equal to about 500 degrees Celsius under conditions that achieve an average concentration in the doped polysilicon film of between about 7xc3x971020 and about 3xc3x971021 boron atoms per cubic centimeter.
According to another aspect of the invention, a method for forming in-situ doped polysilicon comprises providing a surface and substantially simultaneously flowing a first source gas comprising SiH4 and a second source gas comprising BCl3 over the surface at a temperature less than about 500 degrees Celsius under conditions sufficient to achieve in the doped polysilicon an average concentration of between about 7xc3x971020 and about 3xc3x971021 boron atoms per cubic centimeter.
A related embodiment provides for a semiconductor device comprising in-situ doped polysilicon formed by a method comprising providing a surface, and substantially simultaneously flowing SiH4 and BCl3 over the surface at a temperature less than about 500 degrees Celsius, wherein an average concentration of boron atoms in the polysilicon of between about 7xc3x971020 and about 3xc3x971021 per cubic centimeter is achieved.
Another related embodiment provides for a monolithic three dimensional memory comprising polysilicon formed by a method comprising substantially simultaneously flowing SiH4 and BCl3 at a temperature less than or equal to about 500 degrees Celsius, wherein an average concentration of boron atoms in the polysilicon is between about 7xc3x971020 and about 3xc3x971021 per cubic centimeter, wherein the monolithic three dimensional memory comprises two or more memory levels.
A preferred embodiment provides for a method for depositing in-situ doped polysilicon, the method comprising providing a surface comprising a substantially horizontal surface and a substantially vertical sidewall descending from the horizontal surface, the sidewall having a top, and depositing an in-situ doped polysilicon film on the surface at a temperature less than about 500 degrees Celsius, wherein a first thickness of the film at its thinnest point on the vertical sidewall is at least 80 percent of a second thickness of the film on the sidewall at the top of the sidewall, and a third thickness of the film on the horizontal surface is at least 200 angstroms.
Another preferred embodiment provides for an in-situ doped polysilicon film wherein the polysilicon film was deposited at a temperature less than about 500 degrees Celsius, and the polysilicon film is deposited by substantially simultaneously flowing a first source gas comprising SiH4 and a second source gas comprising BCl3, wherein the polysilicon film has a sheet resistance less than about 280 ohms/square.
Other preferred embodiments are provided, and each of the preferred embodiments can be used alone or in combination with one another.
The preferred embodiments will now be described with reference to the attached drawings.